Approximately 9% of shoulder girdle injuries involve the acromioclavicular joint (ACJ). There is no clear gold standard or consensus on surgical management of these injuries, in part perpetuated by our incomplete understanding of native ACJ biomechanics. There is increasing interest in the role of anterior-posterior (AP) instability, which is not reported as often as superior-inferior (SI) displacement from clinical imaging. We have therefore conducted a biomechanical study to measure the amount of restraint provided by each of the stabilizing structures of the ACJ in both SI and AP translation.
Twenty fresh-frozen cadaver shoulder girdle specimens were prepared and mounted in a robotic testing system. The inferior scapula was cemented into a fixed mounting beside the base of the robot. The medial clavicle was cemented into a tubular holder on the end of the moving arm of the robot. The intact native joint was tested in SI and AP translations under 50N displacing force. Each specimen was re-tested after sectioning of its stabilizing structures in the following order: investing fascia, ACJ capsular ligaments, trapezoid ligament, and conoid ligament. Their contributions to resisting ACJ displacements were calculated by measuring the reduction of the force needed to displace the clavicle after each cut. The force contributions of each of the cut structures were tested for statistical significance using repeated measures one-way ANOVA with multiple contrasts and Tukey’s correction. Differences were taken to be significant with p <0.05.
In the intact native ACJ, mean anterior displacement of the clavicle was 7.9 +/- 4.3mm and mean posterior displacement was 7.2 +/- 2.6mm (p=0.652). Mean superior displacement was 5.8 +/- 3.0mm and mean inferior displacement 3.6 +/- 2.6mm (p=0.042) . The conoid ligament was the primary and only significant stabilizer of superior displacement (46%; p<0.001). The ACJ capsular ligament was the primary stabilizer of inferior displacement (34%; p<0.001), but did not provide significant resistance to superior displacement (p=0.167). The capsular ligament and conoid ligament contributed equally to anterior stability, with 23% and 25% respectively (p<0.001). The capsular ligament was the primary contributor to posterior stability (38%; p=0.002) while the conoid ligament contributed 14% (p=0.001).
The conoid ligament is the primary stabilizer of superior displacement of the clavicle at the ACJ and contributes significantly to A and P stability. Consideration should be given to reconstruction of the ACJ capsular ligament for complete AP stability in high grade and horizontally unstable ACJ injuries. These data provide a better understanding of this joint’s complex biomechanical characteristics, thus aiding development and evaluation of reconstructive techniques.